What is the significance of this traceback?
I still don't know how to interpret the traceback, but I found out what triggered it. I exported the elements of the original ~.blend file to ~.dxf format one by one, with all but one of them exporting successfully. The problem triggering the failure of the import turned out to be in one portion of export, where there was one duplicate vertex, which I removed by using the "remove doubles" special menu option. After removing the stray vertex, the formerly misbehaving element exported normally, with no error and no traceback.